zopflipng (with -m switch) is close to or better than the best on those five images:

    1_zopflipng.png  8.466
    2_zopflipng.png 18.739
    3_zopflipng.png  8.012
    4_zopflipng.png 91.879
    5_zopflipng.png  1.117